1. Field of Invention
The invention is related to the field of antennas for receiving radio frequency electromagnetic waves and in particular to a noise rejection antenna system for recreational receivers on nonmetallic marine vessels and other vehicles.
2. Description of the Prior Art
AM-FM receivers using a single antenna input have been well developed for automobiles. However, the use of these same AM-FM receivers in nonmetallic (fiberglass) marine vessels has not been very satisfactory. The major problems encountered are the electric noises generated by the engine's ignition system, electric motors, bilge pumps, electric fans, fluorescent lights, and other similar devices found on marine vehicles. These sources produce electrical noise which is transmitted throughout the vessel and which corresponds in frequency and intensity to the radio signals desired to be received. As a result, the recreational AM-FM receiver is receiving signals from more than one source and the audio output is distorted, has excessive noise and, in general, is of a very poor quality. These same noise sources exist in automotive vehicles, but are easier to control due to the differences in the environment. Automotive manufacturers recognized these problems years ago and have taken a total system approach to remove or suppress most of the noise sources at their origin. In addition, the metal body of the automotive vehicle is an effective electric shield between the noise sources under the hood and in the passenger compartment and the externally mounted antenna.
On nonmetallic marine vessels, the wood flooring and the nonmetallic hulls, such as those made from wood, fiberglass or other plastic materials, are transparent to the electrical noises generated by these sources and the effective electrical shield inherent in the automotive vehicle is nonexistent. As a result, the recreational AM-FM stereo receivers developed for automotive vehicles perform poorly in nonmetallic marine vessels.
The antenna is an important component in determining the performance qualities of the recreational receivers mounted on the marine vessels. A poor antenna can result in a well designed (expensive) recreational receiver having poor performance, while a good antenna can significantly improve the performance of a less expensive recreational receiver.
In automotive vehicles, because of the insulation of the antenna from the sources of noise, a single antenna will normally have adequate reception capabilities for the recreational receiver. The most prominent antenna used in the automotive vehicle is the vertical rod or whip antenna. However, dipole antennas mounted in the windshields of automotive vehicles have also been used extensively. Currently, most automotive vehicle recreational receiver manufacturers utilize a compromise antenna system by using an antenna tuned to the frequency range of the FM radio signals. This antenna also serves as an AM antenna; however, its sensitivity in the AM frequency range is relatively poor. To compensate for the poor sensitivity in the AM frequency range, the manufacturers of the receivers provide additional gain in the RF stages of the receiver.
Preferably, the receiver should have one antenna specifically tuned for the AM radio signal frequency range and another specifically tuned for the FM radio signal frequency range. However, this arrangement alone will not solve the noise problems encountered on nonmetallic marine vessels.
The use of multiple antennas in conjunction with recreational receivers is addressed by Ito, et al., in U.S. Pat. No. 4,193,076. Ito discloses a system for coupling an external rod (whip) antenna with a loop antenna mounted on a magnetic core. In the system taught by Ito, et al., both antennas are tuned to the same frequency range. In a similar manner Garay, et al., in U.S. Pat. No. 4,313,119, disclosed a dual mode transceiver antenna having a low profile loop antenna and a high efficiency dipole antenna. Similarly, Silverstein, in U.S. Pat. No. 3,121,229, discloses an underwater antenna system for a submarine responsive to both the magnetic and electric field components.
Neither Ito, et al., Garay, et al., or Silverstein address the problem of having dual antennas tuned to different frequencies feeding a single antenna input to a recreational receiver. Nor do they address the problems associated with the attenuation of the radiated electrical noises originating from the numerous unshielded sources on a nonmetallic marine vessel.